An exemplary embodiment relates to a radio communication system, a radio base station, a radio communication terminal, and a radio communication method.
In recent radio communication systems, small base stations called femto base stations may be newly introduced in addition to existing base stations, to provide femto cell services. Hereinafter, an existing base station is referred to as a macro base station for the sake of differentiation. Further, in the period of generation changes in radio communication systems, it is expected to provide dual-mode base stations or dual-mode mobile stations corresponding to two old and new radio communication systems, and various apparatuses have been proposed (Japanese Unexamined Patent Application Publication No. 2009-290459 (Nakata)). For example, when femto base stations are developed at the timing of the introduction of an LTE (Long Term Evolution) system, interconnectivity to an existing 3G (3rd Generation) system becomes important, and a femto cell base station forming both cells of the LTE system and the 3G system are required.
Incidentally, the radio communication system typically supports a circuit switching call (CS Call) represented by a voice call and a packet switching call (PS Call). However, since LTE systems at an early stage of introduction cannot address with a VoIP over LTE (VoIP: Voice over Internet Protocol) function that is required for the CS Call, when a dual-mode radio communication terminal using the LTE system makes or receives the CS Call, a CS Fallback function is often supported to forcibly move the terminal from the LTE cell to the 3G cell by a handover. Further, in this case, a function of performing a handover of the radio communication terminal is often supported as well in order to return the terminal from the 3G cell to the LTE cell where higher-speed communication can be achieved when the radio communication terminal ends the CS Call.
Meanwhile, when one base station forms the cell of the LTE system and the cell of the 3G system, coverage areas of the both cells are not normally matched with each other. One reason for this is that both systems use different frequency bands from each other. Even when radio parameters are initially tuned to make the coverage areas of both systems match with each other at the stage of the cell design, there is generated a difference in coverage areas of both systems since the number of users in each cell and radio interference from neighbor cells are different for each system. In particular, in the case of the femto cell, transmission power of a femto base station is adjusted according to propagation loss from the nearest macro base station (Home Node B Radio Frequency (RF) Requirements (FDD) [3GPP TR25.967 v9.0.0]). The location at which the macro base station of the LTE system is installed and the location at which the macro base station of the 3G system is installed are not necessarily the same but are normally different. As a result, the coverage area of the cell of the LTE system and the coverage area of the cell of the 3G system formed by one base station are different.
In general, since it is necessary to perform complicated cell system designing/evaluation work in order to form the service area of the radio communication system, various cell forming methods have been studied. For example, Japanese Unexamined Patent Application Publication No. 2006-135673 (Mori et al.) discloses a mobile communication system that autonomously sets transmission power of a common control channel to form a cell while cooperating with surrounding base stations.
The related arts described above have the following problems.
When a handover caused by movement between cells is performed between cells having different coverage areas, the handover may be ended in failure in some cases. For example, take an example in which the coverage area of the source cell is wider than the coverage area of the destination cell. In this case, when a handover is performed on a radio communication terminal which is in the service area in the source cell but in the outside of the service area in the destination cell, it is expected that communication is not normally performed in the destination cell, which results in failure in the handover. In another case in which a femto base station is used, when a radio communication terminal is located at a position which is outside the service area in a femto cell which is a destination candidate but within the service area in a surrounding macro cell, the radio communication terminal is handed over not to the candidate cell to which the own femto base station was originally intended to move but to a surrounding macro cell, although the handover itself is not unsuccessful. This prevents a user from receiving the benefits of inexpensive communication costs and significant improvement in communication data rate since only a limited number of users are able to use radio resources exclusively. These benefits are something that the user could have received by continuous use of the femto cell, not using the macro cell.
Accordingly, when one base station forms a plurality of cells that use different frequency bands, in order to successfully perform a handover caused by movement between cells with a high probability, it is required to appropriately control coverage areas of the plurality of cells.
The cell forming method disclosed by Mori et al. is a method of forming a cell of an own base station in cooperation with surrounding base stations, and cannot be applied to a case in which it is required to control coverage areas of a plurality of cells that use different frequency bands when one base station forms the plurality of cells.